This invention concerns the field of heat pumps operated with a mixture of working fluids.
The use of working fluids as a mixture, in heat pumps, so as to take heat within a temperature range, and wherein the mixture used in the heat pump is progressively vaporized, in a first heat exchange zone, wherein its temperature is increased with the temperature of an external fluid used as heat source being decreased during said exchange, so as to supply heat within a temperature range, the mixture used in the heat pump being progressively condensed in a second heat exchange zone, wherein its temperature decreases simultaneous with an increase in the temperature of an external fluid to which said heat is transferred. Such process is described in U.S. Pat. No. 4,089,186.
Such a technique provides for a large improvement in the performance coefficient of the heat pumps whenever the temperature of the external fluids with which are the heat exchanges performed varies during said exchanges, and may be used both for industrial applications and house heating.
The use of the working fluid in a heat pump for house heating is of particular interest, and is well adapted to the use of heat pumps operated with mixtures inasmuch as the heats exchanged are always, in this case, at least partly sensible heats and the heat exchanges involve temperature variations of the external fluids with which the exchanges are performed.
The applications which have been described in the U.S. Pat. No. 4,089,186 concern the cases where heat is recovered within a wide temperature range. For this reason, there was described in U.S. Pat. No. 4,089,186, a preferred embodiment consisting of condensing the mixture circulating in a heat pump in two stages, so as to supply heat in a narrower temperature range than the temperature range in which heat is recovered.
On the other hand, water is the only heat source mentioned in the U.S. Pat. No. 4,089,186. As a matter of fact, it is relatively simple to adapt the use of mixtures to heat pump operated with water as external fluid at the evaporator and at the condenser. It is necessary, in the case of a heat pump operated with a fluid mixture, in order to obtain the performance advantages resulting from the use of a mixture, to operate the evaporator and the condenser in a counter-current exchange mode. When these heat exchanges are performed with water, many present day exchangers are adapted to such an exchange mode, and are already operating in such conditions in the case of a single fluid; this is the case, for example, of double-tube exchangers as well as plate exchangers.
On the other hand, when the heat source fluid is air, the evaporator generally operates according to a cross-current heat-exchange mode. The working fluid circulates through tubes having with fins arranged in sheets wherethrough the air is circulated perpendicular to the sheets by means of a fan.
Since it is impossible to recover heat from a water stream under conditions at which water is likely to freeze, the vaporization of the mixture is performed according to the teachings of U.S. Pat. No. 4,089,186, within a temperature range from 0.degree. to 100.degree. C.
In the case of heat pumps operated with air as heat source, when the air contains water vapor, the operation of the evaporator at low temperature produces condensation and frosting of the water contained in the air. This feature results in severe limitations of the conditions under which heat pumps using air as a heat source, when no defrosting device provided for can be used. With the provision of a defrosting device, the efficiency of the heat pump is necessarily decreased.
In the case of a heat pump operated with a single working fluid, the temperature of the evaporating working fluid is substantially uniform, and as soon as the temperature of the air decreases by a few degrees to below 0.degree. C., the water contained in the wet air which is blown onto the evaporator surfaces, condenses and freezes over the surface of the tubes.